The unmarked 18-wheelers ply the nation’s interstates and two-lane highways, logging 3 million miles a year hauling the most lethal cargo there is: nuclear bombs.

The covert fleet, which shuttles warheads from missile silos, bomber bases and submarine docks to nuclear weapons labs across the country, is operated by the Office of Secure Transportation, a troubled agency within the U.S. Department of Energy so cloaked in secrecy that few people outside the government know it exists.

The $237-million-a-year agency operates a fleet of 42 tractor-trailers, staffed by highly armed couriers, many of them veterans of the Iraq and Afghanistan wars, responsible for making sure nuclear weapons and components pass through foggy mountain passes and urban traffic jams without incident.

The transportation office is about to become more crucial than ever as the U.S. embarks on a $1-trillion upgrade of the nuclear arsenal that will require thousands of additional warhead shipments over the next 15 years.

The increased workload will hit an agency already struggling with problems of forced overtime, high driver turnover, old trucks and poor worker morale — raising questions about its ability to keep nuclear shipments safe from attack in an era of more sophisticated terrorism.

“We are going to be having an increase in the movements of weapons in coming years and we should be worried,” said Robert Alvarez, a former deputy assistant Energy secretary who now focuses on nuclear and energy issues for the Institute for Policy Studies in Washington. “We always have to assume the worst-case scenario when we are hauling nuclear weapons around the country.”

That worst case would be a terrorist group hijacking a truck and obtaining a multi-kiloton hydrogen bomb.

“The terror threat is significant,” said one high-level Energy Department official, who spoke on condition of anonymity because he was not authorized to discuss the program publicly. “If you are in one of the communities along the route, you have something to worry about.”

The Los Angeles Times reviewed government documents dating back two decades and interviewed dozens of government officials, former military officers and arms control advocates to examine the agency. The picture that emerges is an organization hampered by an insular management, a crisis of morale among the rank-and-file and outdated equipment.

Among the findings of the investigation:

— The agency is 48 agents short of its planned staffing of 370, a result of budget cuts. Weapons and tactics classes were canceled in 2011 and 2012 for lack of money.

— More than a third of the workforce has been putting in more than 900 hours a year of overtime, which former couriers and Energy Department officials say has contributed to a breakdown in morale and rapid turnover.

— In 2010, an inquiry by the Energy Department’s inspector general inquiry found widespread alcohol problems. It cited 16 alcohol-related incidents over a three-year period, including an agent on a 2007 mission who was arrested for public intoxication and two agents on a 2009 mission who were handcuffed and detained by police after a fight at a bar.

— In 2014, the commander of the agency’s operation at the Y12 National Security Complex in Tennessee threatened to kill an employee in an altercation, but no disciplinary action was taken.

— The agency’s top executive in 2009 was charged with drunk driving after police found him parked on a sidewalk with an open bottle of beer and a blood-alcohol concentration of 0.15%, nearly twice the legal limit, according to New Mexico court records.

— The agency’s truck fleet is antiquated by commercial standards and well past its operational life even under the department’s own guidelines. About half the tractors are more than 15 years old. The high-security trailers used by the agency are even older, designed before the current era of terrorist threats.

How the agency wound up in this state is a story of neglect that begins at the end of the Cold War.

After the Soviet Union broke apart in 1991 and chances of a nuclear attack faded, the U.S. dramatically reduced its nuclear stockpile and gave it less attention as military priorities shifted.

The transportation office budget stagnated, and was hit by big cuts in some years, leading to staffing shortages and delays in updating equipment. Drivers had to start working long hours of overtime, which led to morale problems and management breakdowns.

Despite these problems, the agency asserts that it has maintained a high level of security and has never had lost a weapon, though it has been involved in several accidents.

The agency denied repeated requests for interviews with top managers. It issued a statement touting its safety record: “For more than 40 years — even after driving the equivalent distance of a trip to Mars and back — no cargo has ever been damaged in transit,” it said.

Yet even one of its most stringent security measures was breached, the inspector general found in 2014, when an “unauthorized” employee had access to a nuclear weapon on a convoy mission.

According to two knowledgeable sources, the person in question had lost his human reliability rating, which is based on screening for drugs, alcohol abuse or mental health problems, among other things. Under the agency’s rules, the unidentified employee should not have been allowed on the mission. The employee was discovered at a military base and removed from the assignment.

Overseers in Congress say the transportation office is less prepared for an attack than it used to be.

"It clearly needs a reinvestment,” Rep. Mac Thornberry, the Texas Republican who chairs the House Armed Services Committee, said in an interview. “Like other parts of the nuclear enterprise, the agency has been allowed to atrophy as the country has focused on other things.”

‘Transportation is the Achilles heel of nuclear security’

The United States has 4,018 nuclear warheads.

About 450 are in underground silos in Wyoming, Colorado, Montana, Nebraska and North Dakota. An additional 1,000 or so are on submarines, which dock at bases in Washington and Georgia. Hundreds more bombs are assigned to the U.S. strategic bomber fleet, which is based in Louisiana, North Dakota and Missouri. And a reserve stockpile sits in bunkers near the transportation office headquarters at Kirtland Air Force Base in New Mexico.

Each weapon — a complex physics machine that contains as many as 6,000 parts, including tanks of gas, wheels and gears, batteries, wiring, plastic-type explosives and radioactive materials — requires routine inspection, testing and maintenance.

The workers who perform those services don’t travel to the weapons. The weapons go to them.

They are picked up by the transportation office and driven to the government’s sole plant for working on live nuclear warheads, the Pantex Plant outside Amarillo in the Texas panhandle.

From there, various pieces are parceled out to government plants and laboratories across the country. Uranium assemblies travel to Tennessee, plutonium parts to New Mexico, radioactive gas canisters to South Carolina, non-nuclear classified parts to California and firing mechanisms to Kansas.

Those parts are then returned to Texas so the warheads can be reassembled and trucked back to their silos or military bases.

The system dates back to the 1950s and the rapid buildup of nuclear arms that accompanied the Cold War. Weapons were spread across the nation to ensure that a significant number could not be destroyed in a focused missile strike.

The same went for the facilities that service those weapons. But exactly where they wound up — and where they are today — largely came down to politics, as members of Congress schemed to bring high-paying jobs to their districts.

The result is an unwieldy system that requires some of the most dangerous and vulnerable components of the nation’s defense system to be routinely shipped on long-distance journeys from one end of the country to the other — and the shipments, with the coming modernization effort, are only expected to multiply.

“This has a classic footprint of an antiquated and inefficient supply chain management system that was created at a time of national emergency," said Nick Vyas, an industrial logistics expert at USC.

“If this were a private operation, it would be out of business in less than 90 days,” he said. “No person in their right mind would subscribe to a service like this.”

More serious than the inefficiencies in moving so many parts is the vulnerability inherent in placing nuclear bombs on the highways, several experts said.

“Transportation is the Achilles heel of nuclear security and everyone knows that,” said Bruce Blair, a retired Air Force missile officer, Princeton University researcher and founder of Global Zero, a nonprofit group that seeks elimination of nuclear weapons.

The danger is not a traffic accident — even a fiery crash is not supposed to explode a warhead — but a heist.

“In an age of terrorism, you’re taking a big risk any time you decide to move nuclear material into the public space over long distances via ground transport,” Blair said. “Bad things happen.”

The high-security trailers that carry the weapons present potential intruders with formidable obstacles, including shock-delivering systems, thick walls that ooze immobilizing foam, and axles designed to explode to prevent a trailer from being towed away, according to independent nuclear weapons experts.

“The trucks will kill you,” a scientist involved in the matter said.

The Energy Department recruits ex-soldiers and special operations commandos for its courier jobs, usually veterans of U.S. wars. Incoming agents train for 21 months at Ft. Chafee in Arkansas, focusing on how to counter a roadside attack by terrorists set on stealing a weapon. The couriers must pass yearly psychological and medical assessments.

They spend months each year working out in private gyms, rehearsing tactics and training with high-powered weapons to counter an attack.

The work itself is mundane and tiring, involving long hours on the road under a constant state of high alert. Workers often put in 75 hours a week, according to numerous reviews of the agency.

Matt Hill joined the transportation office after 13 years in the Marine Corps and three deployments to Iraq. He was looking for civilian employment that would tap into his military experience.

But the job was not what Hill expected. Life on the road meant long weeks away from his family. The pay, about $73,000 a year with overtime, was less than he made in the Marines.

Couriers have been quitting, many of them the experienced veterans so crucial to maintaining safety, Hill said. Finally in February 2016, after just three years on the job, Hill quit too.

"The senior agents are all leaving," he said. “People at the top won’t listen.”

‘Ominous symptoms’ of structural problems

The agency has been the target of worker complaints for years.

In the 1990s, a nuclear courier named Jim Bailey alleged that that on-the-job radiation exposure had damaged his DNA and led to birth defects in his daughter.

A panel of experts found that was unlikely. But in a 67-page report issued in 1998, it laid out a number of other deep problems within the agency, finding that “low morale, distrust and poor communications” among agents are “the ominous symptoms of progressively worsening structural problems” in working conditions.

Two-thirds of couriers had symptoms of sleep disorders, including irritability, and the cramped trucks led to knee and back ailments, the report found.

Bailey was fired but sued and won a small amount of back pay and the right to return to his job. He never did.

After the terror attacks of Sept. 11, 2001, the government turned its attention to the nation’s most critical vulnerabilities and concluded that more needed to be done to prevent terrorists from obtaining a nuclear bomb.

In a 2005 letter to Congress, then-Energy Secretary Samuel Bodman called the transportation office a “top priority” and asked Congress for money for more agents, special weapons, tougher armored vehicles and improved tactics.

The goal was to increase staffing from about 290 to 420 couriers by 2008. But the agency never never reached that level, as lawmakers rejected most of the funding request. Today it aims to employ 370 agents but has 332.

The long hours couriers must work — identified as a chronic problem by the inspector general — contribute to poor morale and a tense work environment.

Those tensions can boil over, as when the supervisor in Tennessee threatened to kill an employee. The same supervisor had been involved in seven verbal and physical incidents that weren’t reported, including “uncontrolled anger, hostility and aggression toward fellow workers and authority figures, according to a 2014 report by the inspector general.

The failure to discipline the employee posed a grave danger, the report found, concluding that it raised the risk that “unsuitable individuals could be allowed to protect nuclear weapons, weapon components and special nuclear material, raising possible national security concerns."

As for the alcohol issues cited by the inspector general, the agency has banned beer kegs at parties at the Ft. Chafee dormitory for trainees and mandated random alcohol testing and suspension of agents with a blood alcohol concentration above 0.02%.

And after years of lean budgets — and sometimes outright cuts — the agency requested a 19% increase in fiscal 2017, to $283 million. But Congress didn’t approve it, and the agency’s funding for this year is less than what it received in 2012.

The agency has been able to purchase five new rigs a year. More potent self-defense systems for the trucks are on the way in a trailer dubbed the Mobile Guardian, which the Energy Department is spending $670 million to develop. But the new trailers are not expected to hit the road until 2023 — long after the weapons modernization program is underway.

Meanwhile, the older rigs are well maintained and log fewer miles than comparable commercial trucks, and agency officials are confident they will be able to do the job, said Al Stotts, a spokesman for the nuclear administration.